leak-testing

name	leak-testing
description	Leak testing — methods (pressure/vacuum decay, helium mass spectrometer, bubble, tracer gas, ultrasonic), leak rate units (Pa·m³/s, scc/s, mbar·l/s), sensitivity comparison, ASTM/ISO standards.
metadata	{"priority":7,"promptSignals":{"phrases":["leak testing","leak detection","leak rate","helium leak test","pressure decay test","vacuum leak test","tracer gas leak"],"minScore":3}}

Leak Testing — Complete Skill

Leak Rate Units and Conversions

SI: Pa·m³/s = [pressure × volume / time] Practical: mbar·L/s; scc/s (standard cc/s at 0°C, 1 atm); cc/min; atm·cc/s

Unit	Multiply by to get Pa·m³/s
1 mbar·L/s	0.1
1 scc/s (std cc/s)	0.1013
1 atm·cc/s	0.1013
1 cc/min	1.69 × 10⁻³
1 oz/yr	~8.9 × 10⁻⁸

Leak rate to equivalent hole size: L [Pa·m³/s] = C_d × A × P × √(2γ/(γ-1) × 2/(RT) × ...) (choked flow for large leaks; viscous flow for small pores)

For viscous flow through capillary: Q = π r⁴ ΔP / (8 η L_cap) [Hagen-Poiseuille; r = pore radius]

Sensitivity of Leak Test Methods

Method	Sensitivity [Pa·m³/s]	Notes
Bubble immersion	10⁻² to 10⁻³	Gross leaks only
Pressure gauge decay	10⁻² to 10⁻⁴	Volume dependent
Flow meter	10⁻³ to 10⁻⁵	Large volumes
Thermal conductivity (Pirani)	10⁻⁴ to 10⁻⁵	Rare gas tracers
Ultrasonic detector	10⁻³ to 10⁻⁴	Qualitative
Halogen detector	10⁻⁵ to 10⁻⁷	Refrigerant tracer
Hydrogen detector (H₂/N₂)	10⁻⁶ to 10⁻⁸	Safe H₂ mixture
Helium mass spectrometer	10⁻⁸ to 10⁻¹³	Most sensitive
Radioactive tracer (Kr-85)	10⁻¹³	Special applications

Pressure / Vacuum Decay Testing

Principle: pressurize or evacuate part; measure pressure change vs. time Sensitivity: ΔP = Q × t / V Q_detectable = V × ΔP_resolution / t_test

Example: V = 1 L, ΔP_resolution = 0.01 mbar, t = 30 s: Q_min = 0.001 L × 0.01 mbar / 30 s = 3.3 × 10⁻⁷ mbar·L/s = 3.3 × 10⁻⁸ Pa·m³/s

Differential pressure method: test part vs. reference volume → eliminates temperature drift Temperature stability: 1°C change in 1 L vessel causes 3.4 mbar pressure change at 1 bar → stabilize ≥ 2× thermal time constant before measurement

Masking leaks from connections: differential measurement subtracts known leak paths

Helium Mass Spectrometer Leak Testing

Methods

Bombing (Gross Leak): pressurize sealed part with He; place in vacuum chamber; MSLD measures He escaping through internal He pressure Hard Vacuum (Fine Leak): evacuate part; apply He externally; MSLD detects He entering Sniffer (Pressurized): fill part with He at positive pressure; scan exterior with sniffer probe Accumulation: seal part in bag with He; accumulate for time t; sample bag gas with sniffer

MSLD (Mass Spectrometer Leak Detector): Ionizes gas mixture; magnetic/quadrupole sector separates by m/z; detects He at m/z = 4 Background He in atmosphere: 5.2 ppm → always present as background

Calibrated leak standard: Permeation leak: known Q_ref [Pa·m³/s]; use to calibrate detector response ASTM E498 specifies calibration

Hard Vacuum Test Procedure (ASTM E493)

1. Evacuate test part to P < 1 Pa
2. Connect MSLD; evacuate detector
3. Background reading I_bg [A]
4. Apply He externally to suspected areas
5. Reading rises to I_He
6. Q_part = Q_cal × (I_He - I_bg) / I_cal

Sensitivity: Q_min = 10⁻¹² to 10⁻¹³ Pa·m³/s Limitation: part must withstand vacuum; large volumes slow response

Bubble Leak Testing

Immersion method: pressurize part with air/N₂; submerge in liquid; observe bubbles Sensitivity: ~10⁻³ Pa·m³/s (bubble size ≥ 1 mm/s) Minimum bubble rate: 1 bubble per minute is detectable

Liquid soap application: apply to exterior of pressurized part; watch for bubbles Less sensitive than immersion; used for field checks

Standard: ASME V Article 10; ASTM E515

Tracer Gas Methods

Halogen (Refrigerant) Detection

Heated sensor (Krytox) reacts with halogen → current signal Tracer: HFCs (R-134a, R-1234yf), HCFCs; pressurize part Sensitivity: 10⁻⁵ to 10⁻⁷ Pa·m³/s Applications: refrigeration systems, air conditioning

Hydrogen (H₂/N₂ mixture)

5% H₂ in N₂ — below LEL, non-flammable Semiconductor sensor or thermal conductivity sensor Sensitivity: 10⁻⁶ to 10⁻⁸ Pa·m³/s Applications: automotive cooling circuits, heat exchangers

SF₆ (Sulfur Hexafluoride)

Electron capture detector; extremely sensitive to SF₆ Sensitivity: 10⁻⁸ Pa·m³/s; GWP = 23,500 → restricted under F-Gas Regulation EU 517/2014

Ultrasonic Leak Detection

Turbulent gas flow through leak generates broadband high-frequency noise (> 20 kHz) Ultrasonic detector (Superphonic, SDT270) detects turbulent noise Range: 20–100 kHz; directional probe; 1 m detection distance Sensitivity: ~10⁻³ Pa·m³/s (gross leaks only); use to locate, then quantify with other method Application: compressed air systems, steam traps, valves (during operation — no shutdown required)

Leak Classification (Acceptance Criteria)

Hermetic (electronics, medical implants): MIL-STD-883 Method 1014: fine leak Q < 10⁻⁸ atm·cc/s per unit volume; gross < 1 bubble/30s ASTM F2854: microelectronic packages

Pressure vessels (ASME): No specific numeric leakage; proof test + visual typically For gas service: hydrostatic test at 1.3× MAWP + NDE

Refrigeration (UL, EPA): Annual leakage < 10% of charge (commercial A/C); 30% (industrial) ASHRAE 15: emergency pressure relief required

Medical devices: varies; implants require hermetic (< 10⁻⁹ Pa·m³/s per ISO 20857)

Locating vs. Quantifying

Locating: helium sniffer, soap bubbles, ultrasonic — pinpoints where Quantifying: pressure/vacuum decay, helium MSLD in accumulation mode — total rate Combine both: locate with sniffer, quantify with MSLD chamber

Output

Provide: required leak rate [Pa·m³/s] per spec, recommended test method (with sensitivity), test pressure [bar], stabilization time [s], detection limit at given volume and test time, calibration standard required, applicable standard (ASTM E493/498/515, MIL-STD-883, ASME V), and pass/fail criterion.